Various metals are produced in elemental form from molten salts in high temperature electrolysis cells. For example, magnesium production via electrolysis cells accounts for more than three quarters of all magnesium produced globally. The typical process involves high temperature molten salt electrolysis of MgCl2 in a cell. The process is operated at sufficiently high temperatures to maintain both the electrolyte and the metal in molten states. The process generates liquid magnesium metal and chlorine gas from the salt bath. The lower density magnesium is transported via the cathode to a metal holding chamber, subsequently rising to the salt bath surface. The resultant chlorine gas is removed in order to prevent reversal of the chemical reaction.
Magnesium electrolysis cells that are used in the industry can be classified as sealed cells or unsealed cells, as described in the Peacey et al U.S. Pat. No. 5,565,080. Sealed cells are considered the more modern processing equipment and are tightly sealed to prevent moisture and air from entering the reaction cell. The presence of moist air results in the formation of MgO which will develop MgO-based build up or sludge on the bottom of the cell or which will react with the graphite anodes to digest the graphite and reduce their life expectancy. These cells are designed to operate for an extended period of time without stopping cell operation, and repair or rebuilding, when necessary, is a costly and time consuming process. Such cells include mulitpolar cells as described in the Sivilotti U.S. Pat. No. 4,560,449 and monopolar cells as described in the Andreassen et al U.S. Pat. No. 4,308,116.
FIG. 1 shows a conventional electrolysis cell 10 which comprises an electrolysis chamber 12 and a collection chamber 14, separated by a partition wall 13. Steel cathodes 16 and graphite anodes 18 are provided in the electrolysis chamber. Molten electrolyte flows through a lower opening in the partition wall to the electrolysis chamber and metal flows from the electrolysis chamber through an upper opening in the partition wall and is removed from the collection chamber through an outlet 20. Gas, e.g., chlorine, is removed from the electrolysis chamber through outlet 22.
Refractories are required in the electrolysis cells, for example, in the magnesium electrolysis salt cells, to thermally insulate the bath contents, to prevent failure of the steel containment shell, and to partition zones within the processing cell. With reference to FIG. 1, typical cell construction employs a steel shell 23 and a refractory lining 24. The refractory lining is in the form of an inner wall 26 of super duty firebricks and is in contact with the molten salt bath. Secondary back-up layers of super duty firebrick between the steel shell and the inner firebrick wall 26 which contacts the molten salt bath are also commonly used to control the thermal gradient within the cell and provide a secondary means of containing the molten salt bath. All of these layers are built with refractory brick and moisture-containing mortar. A layer formed of refractory board is also frequently used on the inside of the cell's steel shell to reduce the amount of heat loss.
Typically, the refractory system of an electrolysis cell is formed by laying-up brick work, as well as field-casting monolithic refractory, for example, for forming subhearths and cathode walls. However, various disadvantages result from such construction. For example, the “man-handable” sized brick and small block used in forming the refractory walls for cell construction do not accommodate gaps in their alignment with the steel sheet of the containment shell. Such gaps are typically filled with the same mortar which is used to assemble the bricks and blocks. The mortar has a higher porosity than the other refractory components and therefore is the weakest point in the refractory system. As such, mortar is the preeminent source for leaks and wear in the refractory system. Additionally, the gaps can result in the refractory lining shifting during operation, causing cracking and opening of mortar joints where the electrolyte can infiltrate. Not only is the integrity of the cell compromised, spent cell removal can be difficult when electrolyte has migrated through the brick lining and solidified en mass.
Additionally, casting of the floors, through walls and other components on site with traditional or modern monolithic refractory requires water. The refractory castable is mixed with water, poured, and allowed to set, which can take a period of 12-24 hours, after which water must be removed. The water in the refractory castable consists of both free water, which will evaporate at 212° F., and chemically-bound water of multiphase calcium aluminate hydrates, which is typically liberated over a range of temperatures up to 1150° F. In order to completely remove water from the system, the furnace must be “baked-out” on site before being placed into service. This process may take up to several weeks, and, in practice, it is difficult to completely remove the chemically-bound water. As such, there may be components of the refractory lining which never completely become dehydrated prior to use and can disadvantageously continue to evolve water in service. Further, the subhearth, floor or walls are large components in the electrolysis cell and may contain between 4.5-7% water. The presence of water in such a large amount can create shrinkage cracks upon curing. Once the floor or wall is installed and cured, if cracks are identified, the component may need to be removed and re-poured. On the other hand, if the water is not removed completely before the cell is put into service, it will react to form MgO during the cell operation, which, as noted previously, reduces the operation life and/or the operating efficiency of the cell.
Accordingly, improvements in electrolysis cell refractory construction are desired in order to provide cells which avoid various disadvantages of the prior art.